SI (Systéme International) units:

Length: the meter (m) is defined as the distance light travels in a vacuum in 1/299,792,458 sec (note that this is truly universal: in principle it can be determined by anyone, anytime, anywhere in the Universe).

Mass: the kilogram (kg) however is still based on the International Prototype Kilogram in Paris and the derived standard kilogram standards held by governments around the world.

Time: the second (s) is defined today as the duration of 9,192,631,770 periods of the radiation of two hyperfine levels of the ground state of the cesium 133 atom.

Amount of substance: the mole (mol) is defined as the number of atoms in 0.012 kg (defined, so sig figs not restricted) of carbon 12 atoms.

Temperature: the kelvin (K) is defined as 1/273.15 of the thermodynamic temperature of the triple point of water.

Electric current: the ampere is defined as the the current which carries one coulomb (6.24146 x 10^{18} times the charge on an electron or proton) of charge through a conductor in one second.
You should know (memorize) and be able to interconvert the prefixes in the table below:
SI Prefixes
Prefix 
Symbol 
Magnitude 
tera 
T

10^{12}

giga 
G

10^{9}

mega 
M

10^{6}

kilo 
k 
10^{3} 
base 

10^{0} 
deci 
d 
10^{1} 
centi 
c 
10^{2} 
milli 
m 
10^{3} 
micro 
(or mc) 
10^{6} 
nano 
n 
10^{9} 
pico 
p 
10^{12} 
fempto 
f 
10^{15} 
© R A Paselk
Last modified 30 January 2015